Enhancement of Photocatalytic Activity of ZnO/SiO2 by Nanosized Pt for Photocatalytic Degradation of Phenol in Wastewater

1 Chemistry Department, Faculty of Science, King Abdulaziz University, P.O. Box. 80203, Jeddah 21589, Saudi Arabia 2 Nanostructured Materials and Nanotechnology Division, Advanced Materials Department, Central Metallurgical Research & Development Institute (CMRDI), P.O. Box 87 Helwan, Cairo 11421, Egypt 3 Department of Environmental Sciences, Faculty of Meteorology, Environment, and Arid Land Agriculture, King Abdulaziz University, Jeddah 21589, Saudi Arabia 4 Minerals Technology Department, Central Metallurgical Research & Development Institute (CMRDI), Cairo 11421, Egypt


Introduction
Zinc oxide (ZnO) is an n-type semiconductor with a wide direct bandgap of 3.37 eV.Recently, much effort has been devoted to study ZnO as a promising photocatalyst for photocatalytic degradation of water pollutants, owing to its high activity, low cost, and environmentally friendly feature [1][2][3][4].ZnO microcrystal showed also good photocatalytic activity for dye wastewater treatment [5].
However, a major drawback of ZnO is the large bandgap of 3.37 eV; so, wavelengths below 400 nm are necessary for excitation.Another disadvantage of ZnO is that charge carrier recombination of photogenerated electron/hole pairs occurs within nanoseconds and the photocatalytic activity is low [6][7][8][9].Therefore, it is necessary to improve its visiblelight activities by extending its absorption threshold from the UV light region to the visible light region and also reduce the recombination of photogenerated electron/hole.Different works were performed recently to improve the activity of ZnO catalyst.Development of core/shell-structured materials on a nanometer scale has been receiving extensive attention [10,11].The shell can alter the charge, functionality, and reactivity of surface or improve the stability and dispersive ability of the core material.Furthermore, catalytic, optical, or magnetic functions can be imparted to the core particles by the shell material.In general, the synthesis of core/shell-structured material has the goal of obtaining a new composite material having synergetic or complementary behaviors between the core and shell materials.Many studies on the synthesis of composites such as NiO [12], V 2 O 5 [13], TiO 2 [14], Fe 2 O 3 [15], Pt [16], and Ag [17,18] coated with SiO 2 shells have been reported.SiO 2 is one of the most studied shell candidates due to its relative ease in preparation, good environmental stability, and compatibility with other materials, which motivated us to prepare the core/shellstructured composite of ZnO and SiO 2 and expected to achieve novel properties resulting from the synergic interaction of these two chemical components.One of the most promising methods to increase the photocatalytic efficiency is surface modification of ZnO, this can be achieved by metal doping into the ZnO catalyst.Dopant can act as a sink to collect photogenerated electrons from the conduction band of the semiconductor.Thus, it hinders the recombination of photogenerated electrons and holes through increasing the charge separation [19][20][21][22].The surface modification of ZnO nanoparticles by preparing charge-transfer catalysts with mixing multicomponent oxides can enhance the surface chemical and physical properties and be considered as the key for the successful photocatalytic applications of such nanoparticles.Several metal ions such as Fe [23] and Ag [24,25] have been used as dopants for ZnO to improve its photocatalytic activity.Photocatalytic degradation of phenolic compounds with semiconducting oxides holds promise for the purification and treatment of both drinking and industrial wastewater [26][27][28].The presence of such organic pollutants in aquatic environments has caused several environmental pollution problems.
In the present work, Pt/ZnO-SiO 2 nanoparticles with large specific surface areas had been synthesized by the application of a photoassisted deposition (PAD) and impregnation (Img) methods, and the properties of the nanoparticles were characterized by XRD, TEM, EXAFS, UV-Vis/DRS, and BET analysis.The photocatalytic activity of the synthesized nanoparticles was evaluated by the photodegradation of phenol (as a model for pollutants in wastewater) under UV irradiation.In a typical impregnation (Img) method, the 3 wt% of Pt metal was deposited by a simple impregnation of ZnO-SiO 2 in the absence of light with aqueous solution of H 2 PtCl 6 .The samples were dried at 100 • C and reduced by H 2 (20 mL/min) at 350 • C for 4 h.

Characterization Techniques.
To determine the crystallite sizes and identities of the Pt loaded on ZnO/SiO 2 nanocomposite photocatalyst, X-ray diffraction (XRD) analysis was carried out at room temperature using Rigaku X-ray diffractometer with Cu Kα radiation over a 2θ collection range of 10-80 • .The shape of the samples was tested using Hitachi H-9500 transmission electron microscope (TEM), the prepared samples were prepared by suspending the prepared samples in ethanol, followed by ultrasonication for 30 min, then a small amount of this solution onto a carbon-coated copper grid and drying for TEM.Specific surface area was calculated from measurements of N 2 -adsorption using Nova 2000 series chromatech apparatus at 77 K. Prior to the measurements, all samples were treated under vacuum at 200 • C for 2 h.The band gap of the samples was identified by UV-visible diffuse reflectance spectra (UV-Vis-DRS) in air at room temperature in the wavelength range of 200-800 nm using Shimadzu UV-2450 spectrophotometer.The extended X-ray absorption fine structure (EXAFS) is performed at BL-7C facility [29] of the Photon Factory at the National Laboratory for High Energy Physics, Tsukuba, Tokyo, Japan.A Si (1 1 1) doublecrystal was used to monochromatize the X-rays from the 2.5 GeV electron storage ring.The K-edge EXAFS spectra of Fe were measured in the fluorescence mode at 25 • C. The Fourier transformation was performed on K 3 -weighted EXAFS oscillations in effective range from 0-5 Å.

Photocatalytic Activity Measurements.
Photodegradation experiments were performed with a photocatalytic reactor system at 25 • C.This bench-scale system consisted of a cylindrical Pyrex-glass cell with 1 L. A 150-Watts mercury lamp was placed in a 5 cm diameter quartz tube with one end tightly sealed by a Teflon stopper.The photoreactor was filled with 1 L of 100 mg/L aqueous phenol solution with 1 g/L of each of the three prepared nanoparticles samples.The whole reactor was cooled with a water-cooled jacket on its outside to the temperature.Compressed air was purged into the solution by bubbling compressed air from the bottom to maintain an aerobic condition [30].Magnetic stirrer was also used to keep the solution chemically uniform.The pH of the solutions was adjusted and kept constant at 7 during the experiments by adding NaOH (1 M) and HCl (1 M) using an Orion Model 801 A pH meter.The experiments were carried out for 60 minutes.The liquid samples were filtered for analysis through 0.2 μm syringe filters.The residual phenol was analyzed by the reaction with 4-aminoantipyrine [31].Diluted sample of phenol was treated with 2 mL of 4 N NH 4 OH, 1 mL of 1.5% 4aminoantipyrine, and 1 mL of 4% K 3 [Fe (CN) 6 ] and quantitatively diluted to 100 mL with H 2 O.After 5 min, the reaction was determined colorimetrically at 510 nm.The photodegradation efficiency of phenol has been calculated applying the following equation: where C 0 is the initial phenol concentration; C is the retained phenol in solution.[32].In addition, the intensity of the Pt-Pt peak of the PAD: Pt-ZnO-SiO 2 catalyst is smaller than that prepared by impregnation route.Therefore, Pt metal particles formed on (PAD: Pt-ZnO-SiO 2 ) showed smaller particle size than (Img: Pt-ZnO-SiO 2 ).The grain sizes of PAD: Pt-ZnO-SiO 2 and Img: Pt-ZnO-SiO 2 nanocomposite photocatalysts were displayed in TEM images as shown in Figure 3.The particle size distribution obtained from the analysis of TEM images is shown in Figure 4.The result reveal that the nano-sized Pt metal with a mean diameter (d) of ca. 3 nm having a narrow size distribution was found on the PAD: Pt-ZnO-SiO 2 catalyst, whereas the aggregated Pt metal within various sizes is observed on Img: Pt-ZnO-SiO 2 catalyst (d = 15 nm) which is in agreement with the results of EXAFS measurement.These findings suggest that the size of Pt metal particles depends on the preparation method.Also, TEM micrographs showed the homogenous distribution of Pt over ZnO-SiO 2 matrix which was prepared by PAD method.

Surface Area Analysis. Specific surface area (S BET ) of
ZnO/SiO 2 , PAD: Pt-ZnO-SiO 2 , and Img: Pt-ZnO-SiO 2 powder samples were determined.The S BET values were 480, 460, and 450 m 2 /g for the ZnO-SiO 2 , PAD: Pt-ZnO-SiO 2 , and Img: Pt-ZnO-SiO 2 , respectively.The parameters of surface area and the data calculated from the t-plot are collected in Table 1.Furthermore, the total pore volume of Pt-ZnO-SiO 2 is higher than that of ZnO-SiO 2 .The values of S BET and S t are generally close in most samples indicating the presence of mesopores.The values of S micro are high compared to that of S meso implying that the main surface is mesoporous solid.The surface texture data are correlated with the catalytic activity as will be mentioned later on.The N 2 adsorptiondesorption isotherm of the prepared samples is shown in Figure 5.It is clear that the isotherm shows a typical type IV sorption behavior, which confirms the presence of mesoporous form.

Band-Gap Analysis (UV-Vis-DRS).
Study of the UVvisible radiation absorption is an important tool for evaluating the changes in the absorbance spectra of the prepared semiconductor materials.This is expressed by the band-gap (E g ) measurement which can be altered by different parameters.For instance, E g value for pure ZnO phase is usually reported 3.37 [7]; however, these values are influenced by the synthesis method, and also affected by the existence of impurities doping the crystalline network and also the average crystal size of the semiconductor.In a previous study, different methods for calculating the E g from the UV-Vis reflectance spectra were used.For example, some authors calculated the E g values by a direct extrapolation of the F(R) spectrum, whereas others reported the wavelength corresponding to the maximum absorption [33].As a consequence, quite different E g values for ZnO samples are found in the literature.For instance, threshold wavelength values of 240 nm [34], 290 nm [35], and 360 nm [36] correspond to bandgaps 5.15, 4.28, and 3.45, respectively.Figure 6 gives UV-Vis-DRS of (ZnO-SiO 2 , Img: Pt-ZnO-SiO 2 , and PAD: Pt-ZnO-SiO 2 ).The results showed an increase in the absorbance in the visible light region with the Pt doping.The values of E g for the synthesized semiconductors can be derived from the spectra by plotting (F(R) • hν) 1/2 against hν [37,38] as shown in Figure 7 and tabulated as shown in Table 2.The results revealed that the calculated values of E g for (ZnO-SiO 2 , Img: Pt-ZnO-SiO 2 , and PAD: Pt-ZnO-SiO 2 ) were 3.4, 3.25, and 3.05 eV, respectively.This indicates shifting the spectra of the PAD: Pt-ZnO-SiO 2 sample to the visible light area.

Evaluation of Photocatalytic Activity.
The photocatalytic activity of the synthesized nanoparticles samples was evaluated by degradation of phenol in solution under UV light.Figure 8 displays the photocatalytic degradation of Note: (S BET ) BET-Surface area, (S t ) surface area derived from V l−t plots, (S mic ) surface area of micropores, (S ext ) external surface area, (V p ) total pore volume, (V mic ) pore volume of micropores, (V mes ) pore volume of mesopores, and (r − ) mean pore radius.change in the photodegradation activity may be explained in terms of the differences in interaction between Pt and ZnO-SiO 2 that led to several modifications in physical properties such as bandgap, particle size, and surface texture.Also, one could observe that the catalytic activity of ZnO-SiO 2 generally increased with the addition of Pt promoters.
Figure 9 shows the good correlation between the physical properties of the synthesized samples, such as bandgap, surface area, and pore volume, with their catalytic activity.It was obvious that the photodegradation activity was gradually increased with the decrease of bandgap and the increase of the surface area and pore volume.The maximum photocatalytic degradation of phenol was achieved in the case of PAD: Pt-ZnO-SiO 2 in which the surface area and pore volume were maximum with lower bandgap value.It is believed that the lack of electron scavengers (surface Zn 2+ ) and hole traps (surface hydroxyl groups) is responsible for the rapid recombination rate of e − /h + , which leads to lower photocatalytic activity with the parent ZnO-SiO 2 sample [6][7][8][9].The photocatalytic activities of the Pt-doped ZnO-SiO 2 nanoparticles increased due to that Pt plays two important roles; noble metal incorporation into TiO 2 dielectric provides an absorption feature due to the surface plasmon resonance (SPR) occurring over the visible range of the spectrum.In particular, Ag, Pt, and Au metals are the most popular materials due to the strong SPR character [39,40].On the other hand, the superior driven photocatalytic efficiency of the Pt/TiO 2 nanocomposite photocatalyst can be ascribed to the high efficiency of charge-pair separation due to the presence of deposited Pt serving as electron sinks to retard the rapid e − -h + couple recombination, the good photoabsorption capacity in the visible light region, and the higher concentration of surface hydroxyl groups, which are able to effectively scavenge photogenerated valence band holes [41].
International Journal of Photoenergy  Accordingly, the highest activity was more clear in the PAD: Pt-ZnO-SiO 2 sample in which the Pt diffuses in the lattice of the semiconductor.Thus, it lowered both the bandgap (as confirmed from the UV-Vis-DRS spectra analysis) and particle size (as confirmed from the TEM analysis).This resulted in increase in the surface area and pore volume, and consequently showed the best photoactivity with phenol degradation.

Conclusions
Pt doping, through photoassisted deposition (PAD) and impregnation (Img) routes, can greatly enhance the performance of ZnO-SiO 2 as a photocatalyst.The nano-sized Pt metal with a mean diameter (d) of ca. 3 nm having a narrow size distribution was found on the PAD: Pt-ZnO-SiO 2 catalyst, whereas the aggregated Pt metal with various sizes is observed on Img: Pt-ZnO-SiO 2 catalyst (d = 15 nm).The surface area of the synthesized samples was decreased from 480 to 460 and 450 m 2 /g with ZnO-SiO 2 , PAD: Pt-ZnO-SiO 2 , and Img: Pt-ZnO-SiO 2 , respectively.The UV-Vis-DRS spectra analysis confirmed the lowest bandgab of PAD: Pt-ZnO-SiO 2 with a value of 3.05, comparing to 3.25 and 3.4 with Img: Pt-ZnO-SiO 2 and ZnO-SiO 2 , respectively.The photocatalytic degradation of phenol in synthetic wastewater solution was found to be much more effective in the PAD: Pt-ZnO-SiO 2 .The degradation efficiency increased from 80 to 85 and 99.9%, with the ZnO/SiO 2 , Img: Pt-ZnO-SiO 2 , and PAD: Pt-ZnO-SiO 2 samples, respectively.The smallest particle size and lowest bandgab of the PAD: Pt-ZnO-SiO 2 sample resulted in a high increase in the surface area and pore volume and consequently showed the best photoactivity with phenol degradation.

Figure 9 :
Photocatalytic degradation of phenol (%) The XRD patterns of the ZnO-SiO 2 and Pt-doped ZnO-SiO 2 nanoparticles prepared by (Img) and (PAD) routes are shown in Figure1.It can be seen that the diffraction patterns of ZnO-SiO 2 sample and all Pt-doped ZnO-SiO 2 are mainly composed of ZnO phase which still exists after applying both mentioned preparation methods.While in the Pt-doped samples, no diffraction peaks of Pt were observed, this is probably attributed to the low Ptdoping content (ca. 3 wt%).Moreover, it is obvious that Pt is well dispersed over the ZnO-SiO 2 surface.3.2.Nanostructure Characterization.Figure 2 displays the Fourier transforms of Pt L III -edge EXAFS spectra of the Ptloaded catalysts.It can be noticed that the presence of the peak assigned to the Pt-Pt bond of Pt metal at around 2.5 Å is an indication of the formation of nanosized Pt metal 3.1.Phase Analysis.